彩色印刷品常采用CMYK四種原色油墨，以不同的網(wǎng)點(diǎn)比例印刷在紙上，就得到了各種印刷色彩。為了檢驗(yàn)印刷品的質(zhì)量，就可以使用密度儀對印刷油墨密度、網(wǎng)點(diǎn)進(jìn)行測量。

  印刷色彩與網(wǎng)點(diǎn)是什么？

  印刷色譜是印刷行業(yè)顏色系統(tǒng)。彩色印刷通常采用青（C）、品紅（M）、黃（Y）和黑（K）四種原色油墨，這四種原色油墨以不同的網(wǎng)點(diǎn)比例印刷在紙上，就得到了各種印刷顏色，各種印刷網(wǎng)點(diǎn)形成顏色的總和就構(gòu)成了印刷顏色系統(tǒng)，通常有稱為CMYK顏色空間，因此印刷的顏色系統(tǒng)也是一種混色系統(tǒng)。在印刷制作過程中和設(shè)計(jì)印刷版面時(shí)，經(jīng)常需要知道印刷特定顏色時(shí)所需要的原色網(wǎng)點(diǎn)比例，也經(jīng)常需要知道所設(shè)計(jì)的原色網(wǎng)點(diǎn)比例印刷后是什么效果，在這種情況下都需要對照印刷色譜來確定顏色。

  油墨在紙張上以網(wǎng)點(diǎn)的形式存在，各網(wǎng)點(diǎn)之間有兩種個(gè)相互的存在關(guān)系：網(wǎng)點(diǎn)疊合和網(wǎng)點(diǎn)并列。疊合部分的網(wǎng)點(diǎn)以色料減色法原理形成顏色，并列部分的網(wǎng)點(diǎn)以色料減色法原理和色光加色法原理呈現(xiàn)顏色。網(wǎng)點(diǎn)疊合形成的顏色又與某一網(wǎng)點(diǎn)并列呈現(xiàn)新的顏色。網(wǎng)點(diǎn)疊合發(fā)生在中間調(diào)以上的暗調(diào)區(qū)域，而網(wǎng)點(diǎn)并列主要發(fā)生在中間調(diào)以下的亮調(diào)區(qū)域。

  當(dāng)黃、品紅、青三種原色油墨互相疊印時(shí)，形成二次色紅、綠、藍(lán)和三次色黑。因此網(wǎng)點(diǎn)疊合形成二次色和三次色，網(wǎng)點(diǎn)并列時(shí)，反射回的色光即該網(wǎng)點(diǎn)原色油墨的顏色。并列后所形成的新顏色與其他網(wǎng)點(diǎn)顏色之間仍然是并列的關(guān)系。當(dāng)網(wǎng)點(diǎn)非常小，觀察的距離又足夠大，眼睛不能觀察到單獨(dú)的小網(wǎng)點(diǎn)存在，就發(fā)生了顏色相加的混色效果，各種顏色小網(wǎng)點(diǎn)根據(jù)比例形成各種各樣的混合色。

  如何檢測印刷品的質(zhì)量？

  在印刷中，GATF彩色六角形中，互不相鄰的三個(gè)角上是理想的三原色（YMC），另外三個(gè)角上是疊印的復(fù)合色（RGB），中心的為中性色。彩色六角形的中心密度為零，自中心向頂點(diǎn)密度逐級(jí)增加，通過C、M、Y、R、G、B的位置關(guān)系可以直觀地判斷色相偏移等現(xiàn)象。此外，采用了25%、50%、75%和實(shí)地四個(gè)階梯組成的4個(gè)環(huán)進(jìn)行研究，通過它們之間的相對位置關(guān)系，可以直觀地判斷印刷反差等屬性。

  基于密度的印刷品質(zhì)量評(píng)測技術(shù)是目前用于印刷品質(zhì)量檢測與控制的主要手段之一，通過使用反射密度計(jì)測量不同網(wǎng)點(diǎn)覆蓋率下的網(wǎng)點(diǎn)密度和實(shí)地密度，采用顏色六角形對印刷品質(zhì)量進(jìn)行評(píng)測。

  密度計(jì)對四色印刷品的密度測量：

  對于單色測量來說，單色密度儀的設(shè)計(jì)，只需使用一個(gè)符合明視覺光譜光效率函數(shù)V(λ)濾即可，所測密度值為視覺密度。彩色印刷測量的是彩色密度，需要獲得圖像上某一色相的濃淡層次的定量表示，即顏色的飽和度。由于彩色印刷采用紅、綠、藍(lán)的補(bǔ)色青、品紅、黃來控制進(jìn)入眼睛的紅、綠、藍(lán)光的數(shù)量，用光學(xué)密度表示彩色，就是要記錄下被測物體對紅/綠/藍(lán)三種光線的透過或反射的光量，換算成物體的反射或透射率，取其倒數(shù)的負(fù)對數(shù)，來表示顏色的深淺，反射或透過的光量不同，相應(yīng)的紅/綠/藍(lán)密度數(shù)據(jù)不同。能夠?qū)崿F(xiàn)這種測量的儀器是光學(xué)密度儀，稱為彩色密度儀。用它可以測量印刷品的分色密度。彩色密度計(jì)分為透射密度儀和反射密度儀兩類，透射密度儀用于測量膠片上的密度;反射密度儀可以測量的是樣品對紅、綠、藍(lán)光的吸收量，因此在光電探測器前要分別放置紅、綠、藍(lán)和視覺密度濾，用來分別透過紅、綠、藍(lán)光，或者將濾置于光源后面，用來產(chǎn)生紅、綠、藍(lán)照明光。由于青、品紅、黃油墨分別吸收紅、綠、藍(lán)光，并且墨層厚度越大，對紅、綠、藍(lán)光的吸收就越多。所以，測量經(jīng)過油墨吸收后剩余的紅、綠、藍(lán)光量，就可以得到油墨的密度值，間接得到墨層的厚度。

      Color prints often use CMYK four primary color inks and print them on paper with different dot ratios to obtain various printing colors. In order to check the quality of printed matter, a densitometer can be used to measure the density and dots of the printing ink.
  What are the printing colors and dots?
  Printing color spectrum is the color system of the printing industry. Color printing usually uses four primary color inks of cyan (C), magenta (M), yellow (Y) and black (K). These four primary color inks are printed on paper with different dot ratios to obtain a variety of printing Color, the sum of the colors formed by various printing dots constitutes the printing color system, which is usually called the CMYK color space, so the printed color system is also a color mixing system. In the printing production process and when designing the printing layout, it is often necessary to know the primary color dot ratio required for printing a specific color, and it is often necessary to know the effect of the designed primary color dot ratio after printing. In this case, you need to control printing. Color spectrum to determine color.
  Ink exists in the form of dots on the paper, and there are two mutual relations between the dots: dot overlap and dot juxtaposition. The dots of the superimposed part form colors based on the principle of pigment subtractive method, and the dots of the parallel part show colors based on the principle of pigment subtractive color method and color light additive method. The color formed by the overlapping of the dots and a certain dot side by side presents a new color. The overlap of dots occurs in the dark areas above the midtones, while the juxtaposition of dots mainly occurs in the bright areas below the midtones.
  When the three primary colors of yellow, magenta, and cyan ink are superimposed on each other, the secondary colors red, green, blue and the tertiary black are formed. Therefore, the dots are superimposed to form the secondary color and the tertiary color. When the dots are aligned, the reflected color light is the color of the primary ink of the dot. The new color formed after juxtaposition is still in juxtaposition with the colors of other dots. When the dots are very small and the observation distance is large enough, the eyes cannot observe the existence of individual small dots, and the color mixing effect of color addition occurs. The small dots of various colors form a variety of mixed colors according to the proportion.
  How to check the quality of printed matter?
  In printing, in the GATF color hexagon, the three non-adjacent corners are ideal three primary colors (YMC), the other three corners are overprinted composite colors (RGB), and the center is a neutral color. The center density of the colored hexagon is zero, and the density increases step by step from the center to the vertex. The hue shift and other phenomena can be intuitively judged through the positional relationship of C, M, Y, R, G, and B. In addition, 4 rings composed of 25%, 50%, 75% and four steps on the ground are used for research. Through the relative position relationship between them, the printing contrast and other attributes can be judged intuitively.
  Density-based printing quality evaluation technology is currently one of the main methods used for printing quality inspection and control. It uses a reflection densitometer to measure dot density and solid density under different dot coverage, and uses color hexagons to evaluate printing quality.
  Density measurement of four-color printed matter by density meter:
  For monochromatic measurement, the design of the monochromatic densitometer only needs to use a filter that conforms to the optical efficiency function V(λ) of the photopic vision spectrum, and the measured density value is the visual density. Color printing measures color density, and it is necessary to obtain a quantitative expression of the shades of a certain hue on the image, that is, the saturation of the color. Since color printing uses the complementary colors of red, green, and blue, cyan, magenta, and yellow to control the amount of red, green, and blue light entering the eyes, the optical density is used to express the color, which is to record the red/green/blue three The amount of transmitted or reflected light of a kind of light is converted into the reflection or transmittance of the object, and the negative logarithm of the reciprocal is taken to indicate the depth of the color. The amount of light reflected or transmitted is different, and the corresponding red/green/blue density data different. The instrument that can achieve this measurement is an optical densitometer, called a color densitometer. It can be used to measure the color separation density of printed matter. Color densitometers are divided into transmission densitometers and reflection densitometers. The transmission densitometer is used to measure the density on the film; the reflection densitometer can measure the absorption of red, green and blue light by the sample, so it is in front of the photodetector Place the red, green, blue, and visual density filters separately to transmit red, green, and blue light separately, or place the filters behind the light source to produce red, green, and blue illuminating light. Since the cyan, magenta, and yellow inks absorb red, green, and blue light respectively, and the greater the thickness of the ink layer, the more red, green, and blue light are absorbed. Therefore, by measuring the remaining amount of red, green, and blue light after the ink is absorbed, the density value of the ink can be obtained, and the thickness of the ink layer can be obtained indirectly.